This invention relates in general to vehicle power steering systems and in particular to a variable orifice control valve which varies the amount of assistance provided by a power steering system.
Power steering systems are included in many vehicles to reduce the effort required by the vehicle operator to steer the vehicle. Such systems usually include a hydraulically activated actuator which provides assistance in moving the vehicle steering linkage when the steering wheel is rotated. The amount of assistance provided is a function of the steering torque exerted upon the steering wheel by the vehicle operator with the amount of assistance increasing as the operator exerts an increased steering torque. In the past, for any given amount of steering torque, the same amount of assistance has been provided. Recently, however, variable assist power steering systems have been developed which vary the amount of assistance provided for a given amount of steering torque as a function of a vehicle operating parameter, such as vehicle speed. Such systems provide greater assistance for low speed maneuvers, such as parking and less assistance at high speeds where the assistance can detract from the operator's feel of the road.
A typical variable assist hydraulic power steering system is illustrated generally at 10 in FIG. 1. The vehicle steering wheel 11 is coupled to a primary rotary control valve 12 which closes at a low steering torque. A torsion bar 13 couples the primary rotary control valve 12 to a secondary rotary control valve 14 which closes at a relatively high steering torque.
The primary rotary control valve 12 receives pressurized hydraulic fluid from a pump 15. An orifice 16 and relief valve 17 are connected between the pump 15 and the primary rotary control valve 12. The orifice 16 and relief valve 17 are operable to maintain a constant rate of flow for the fluid supplied from the pump 15. The relief valve 17 discharges into a fluid reservoir 18 which is typically located adjacent to the pump 15. The reservoir 18 supplies hydraulic fluid to the pump 15 (not shown).
The primary rotary control valve 12 is connected to a hydraulically activated actuator 19 and is operative to supply hydraulic fluid to the actuator 19 at a pressure which is a function of the torque applied to the steering wheel 11 by the vehicle operator. The actuator 19 is coupled to the vehicle steering linkage (not shown) and is operable in a known manner to move the steering linkage. Hydraulic fluid is discharged from the primary rotary control valve 12 and returned to the reservoir 18 through a hydraulic line.
The secondary rotary control valve 14 also receives pressurized hydraulic fluid from the pump 15. A control valve 20 is connected between the secondary rotary control valve 14 and the pump 15. The control valve 20 has a variable orifice and is operable to vary the volume of hydraulic fluid flowing to the secondary rotary control valve 14. The control valve 20 is actuated by an electronic control module (ECU) 21 which receives a vehicle speed signal from a speed sensor 22. The control valve 20 is responsive to the electronic control module 21 to open the valve orifice as the vehicle speed increases and to close the valve orifice as the vehicle speed decreases. Hydraulic fluid is discharged from the secondary rotary control valve 14 and returned to the reservoir 18 through a hydraulic line.
During operation of the power steering system 10, opening the control valve orifice adds a portion of the secondary rotary control valve area to the area of the primary rotary control valve increasing the total effective rotary control valve area of the power steering system 10. Because the pump 15 supplies the hydraulic fluid at a constant flow rate, the larger effective rotary control valve area reduces the pressure of the hydraulic fluid supplied to the actuator 19. This decreases the amount of assistance provided to the vehicle operator and increases the stiffness of the feel of the steering system 10. When the control valve orifice is fully open, the total rotary control valve area is essentially the sum of the primary and secondary rotary control valve areas.
Conversely, closing the control valve orifice decreases the total effective rotary control valve area, raising the pressure of the hydraulic fluid supplied to the actuator 19. This increases the amount of assistance provided to the vehicle operator and decreases the stiffness of the feel of the power steering system 10. When the control valve orifice is fully closed, the effective area of the rotary: control valves 12 and 14 becomes that of only the primary rotary control valve 12.
A known control valve having a variable orifice is shown generally at 25 in FIG. 2. The control valve 25 includes a valve housing 26 having a generally cylindrical valve chamber 27 formed therein. The valve chamber 27 communicates with an inlet port 28 which is connected to the pump 15. The valve chamber 27 also communicates with an outlet port 29 which is connected to the secondary rotary control valve 14. A first end of the valve chamber 27 receives a threaded plug 30 which has an axial bore 31 formed therein. The plug 30 also has an annular recess 32 formed therein. A transverse bore 33 extends from the annular recess 32 through the plug 30 and communicates with the inlet port 28.
An axially shiftable valve spool 34 is disposed in the axial bore 31 of the plug 30. The valve spool 34 has an axial bore 35 formed therethrough. The valve spool 34 also has a first transverse bore 36 having a relatively small diameter and a second transverse bore 37 having a relatively large diameter which extend through the axial bore 35. As shown in FIG. 2, the plug 30 blocks communication between the first transverse bore 36 and the inlet port 28 while the second transverse bore 37 communicates with the outlet port 29. When the valve spool 34 is shifted to the right in FIG. 2, the first transverse bore 36 cooperates to with the annular recess 32 in the plug 30 to define a variable orifice which provides communication between the inlet and outlet ports 28 and 29.
The valve spool 34 is connected to a shaft 38 of a stepper motor 39. The stepper motor 39 is electrically connected to the electronic control module 21. The stepper motor 39 is responsive to signals generated by the control module 21 to rotate the shaft 38 a predetermined amount. As the shaft 38 rotates, the shaft 38 also moves axially into or out of the valve chamber 27. The axial movement of the shaft 38 shifts the valve spool 34 within the valve chamber 27 to vary the size of the orifice formed between the first transverse bore 36 and the annular recess 32.
As illustrated in FIG. 2, the valve spool 34 is extended to the left, closing the orifice and blocking the flow of hydraulic fluid to the secondary rotary control valve 14. Accordingly, the associated power steering system 10 is operable with only the primary rotary control valve 12 and maximum assistance is provided to the vehicle operator. This condition corresponds to low speed vehicle operation.
The electronic control module 21 is responsive to an increase in vehicle speed to supply an electrical signal to the stepper motor 39. The signal causes the stepper motor 39 to rotate, shifting the shaft 38 and valve spool 34 to the right in FIG. 2. As the valve spool 34 shifts to the right, the orifice formed between the first valve spool transverse bore 36 and the plug annular recess 32 is opened, allowing hydraulic fluid to flow through the control valve 25 to the secondary rotary control valve 14. As explained above, the flow of hydraulic fluid to the secondary rotary control valve 14 reduces the pressure of the hydraulic fluid supplied to the actuator 19. This decreases the amount of assistance provided by the power steering system 10 and increases the stiffness of the power steering system 10. As the vehicle speed further increases, the stepper motor 39 shifts the valve spool 34 further to the right, exposing a greater area of the orifice. This increases the flow of hydraulic fluid through the control valve 25 which further reduces the pressure of the hydraulic fluid supplied to the actuator 19. Accordingly, the amount of assistance provided by the power steering system 10 is further reduced and the stiffness of the power steering system 10 is further increased.
When the vehicle speed is reduced, the electronic control unit 21 causes the stepper motor 39 motor to rotate in a reverse direction shifting the armature 38 and the valve spool 34 to the left in FIG. 2. The shift of the valve spool 33 reduces the size of the orifice which restricts the flow of hydraulic fluid to the secondary rotary control valve 14. This increases the pressure of the hydraulic fluid supplied to the actuator 19, increasing the amount of assistance provided by the power steering system 10 and decreasing the stiffness of the power steering system 10.